Free-cutting steel for machine structural use having good machinability in cutting by cemented carbide tool

Abstract

Disclosed is a free-cutting steel for machine structural use which always exhibits desired machinability, particularly, machinability by cutting with cemented carbide tools. This free-cutting steel is produced by preparing a molten alloy of the composition consisting essentially of, by weight %, C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5% and O: 0.0005-0.01%, the balance being Fe and inevitable, and adjusting the addition amounts of Al and Ca in such a manner as to satisfy the above ranges, S: 0.01-0.2%, Al: 0.001-0.020% and Ca: 0.0005-0.02%, and the conditions of [S]/[O]: 8-40 [Ca]×[S]: 1×10 −5 −1×10 −3 [Ca]/[S]: 0.01-20 and [Al]: 0.001-0.020% to obtain a steel characterized in that the area in microscopic field occupied by the sulfide inclusions containing Ca of 1.0% or more neighboring to oxide inclusions containing CaO of 8-62% is 2.0×10 −4 mm 2 or more per 3.5 mm 2 .

What is claimed is:  
     
       1. A free-cutting steel for machine structural use consisting essentially of, by weight %, C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5%, S: 0.01-0.2%, Al: 0.001-0.019%, Ca: 0.0005-0.02% and O: 0.0005-0.01%, the balance being Fe and inevitable impurities, and is characterized in that the area in microscopic field occupied by the sulfide inclusions containing Ca of 1.0% or more neighboring to oxide inclusions containing CaO of 8-62% is 2.0×10 −4  mm 2  or more per 3.5 mm 2 . 
     
     
       2. The free-cutting steel according to  claim 1 , wherein the steel further contains, in addition to the alloy components set forth in  claim 1 , one or more of Cr: up to 3.5%, Mo: up to 2.0%, Cu: up to 2.0%, Ni: up to 4.0% and B: 0.0005-0.01%. 
     
     
       3. The free-cutting steel according to  claim 1 , wherein the steel further contains, in addition to the alloy components set forth in  claim 1 , one or more of Nb: up to 0.2%, Ti: up to 0.2%, V: up to 0.5% and N: up to 0.04%. 
     
     
       4. The free-cutting steel according to  claim 1 , wherein the steel further contains, in addition to the alloy components set forth in  claim 1 , one or more of Ta: up to 0.5%, Zr: up to 0.5% and Mg: up to 0.02%. 
     
     
       5. The free-cutting steel according to  claim 1 , wherein the steel further contains, in addition to the alloy components set forth in  claim 1 , one or more of Pb: up to 0.4%, Bi: up to 0.4%, Se: up to 0.4%, Sn: up to 0.1%, Sb: up to 0.1% and Ti: up to 0.05%. 
     
     
       6. A method of producing the free-cutting steel for machine structural use having good machinability in machining with a cemented carbide tool set forth in  claim 1 , comprising the steps of preparing an alloy consisting essentially of, by weight %, C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5% and O: 0.0005-0.01%, the balance being Fe and inevitable impurities by melting and refining process for the conventional steel making, and adjusting the addition amounts of Al and Ca in such a manner as to satisfy the above ranges, S: 0.01-0.2%, Al: 0.001-0.019%, and Ca: 0.0005-0.02%, and the conditions of 
       [S]/[O]: 8-40  
       [Ca]×[S]: 1×10 −5 −1×10 −3    
       [Ca]/[S]: 0.01-20 and  
       [Al]: 0.001-0.019%.  
     
     
       7. A method of producing the free-cutting steel for machine structural use having good machinability in machining with a cemented carbide tool set forth in  claim 2 , comprising the steps of preparing an alloy consisting essentially of, by weight %, C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5% and O: 0.0005-0.01%, and further, one or more of Cr: up to 3.5%, Mo: up to 2.0%, Cu: up to 2.0%, Ni: up to 4.0% and B: 0.0005-0.01%, the balance being Fe and inevitable impurities by melting and refining process for the conventional steel making, and adjusting the addition amounts of Al and Ca in such a manner as to satisfy the ranges of S, Al and Ca, and the conditions set forth in  claim 6 . 
     
     
       8. A method of producing the free-cutting steel for machine structural use having good machinability in machining with a cemented carbide tool set forth in  claim 3 , comprising the steps of preparing an alloy consisting essentially of, by weight %, C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5% and O: 0.0005-0.01%, the balance being Fe and inevitable impurities by melting and refining process for the conventional steel making, adjusting the addition amounts of Al and Ca in such a manner as to satisfy the ranges of S, Al and Ca, and the conditions set forth in  claim 6 , and finally, adding one or more of Nb: up to 0.2%, Ti: up to 0.2%, V: up to 0.5% and N: up to 0.04%. 
     
     
       9. A method of producing the free-cutting steel for machine structural use having good machinability in machining with a cemented carbide tool set forth in  claim 4 , comprising the steps of preparing an alloy consisting essentially of, by weight %, C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5% and O: 0.0005-0.01%, the balance being Fe and inevitable impurities by melting and refining process for the conventional steel making, adjusting the addition amounts of Al and Ca in such a manner as to satisfy the ranges of S, Al and Ca, and the conditions set forth in  claim 6 , and finally, adding one or more of Ta: up to 0.5%, Zr: up to 0.5% and Mg: up to 0.02%. 
     
     
       10. A method of producing the free-cutting steel for machine structural use having good machinability in machining with a cemented carbide tool set forth in  claim 5 , comprising the steps of preparing an alloy consisting essentially of, by weight %, C: 0.05-0.8%, Si: 0.0 1-2.5%, Mn: 0.1-3.5% and O: 0.0005-0.01%, and further, at least one of Pb: up to 0.4%, Bi: up to 0.4%, Se: up to 0.4%, Sn: up to 0.1% and Ti: up to 0.05%, the balance being Fe and inevitable impurities by melting and refining process for the conventional steel making, and adjusting the addition amounts of Al and Ca in such a manner as to satisfy the ranges of S, Al and Ca, and the conditions set forth in  claim 6 .